Datasheet LT3740 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | Wide Operating Range, Valley Mode, No RSENSE Synchronous Step-Down Controller |
| Pages / Page | 20 / 8 — APPLICATIONS INFORMATION. Current Sensing Range. Reverse Current Limit. … |
| File Format / Size | PDF / 244 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Current Sensing Range. Reverse Current Limit. Table 1. Current Sensing Thresholds. RANGE PIN
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LT3740
APPLICATIONS INFORMATION Current Sensing Range Reverse Current Limit
Inductor current is determined by measuring the voltage Because the LT3740 operates in forced continuous mode across a sense resistance – either the on-resistance of when the feedback voltage is higher than 720mV, the the bottom MOSFET or an external sensing resistor. The inductor current can go negative on occasion, such as maximum current sense threshold has three steps that are light load, shutting down with a slow SHDN signal, large selected by the RANGE pin. The current sense threshold load step-down transient response, or the output voltage voltage without slope compensation is shown in Table 1. being pulled up by some other power supply. The LT3740 This is the value for high duty cycle operation. has a reverse current comparator to limit the reverse current. During the on-time of the bottom MOSFET, when
Table 1. Current Sensing Thresholds
(VSN+)–(VSN–) reaches 40mV, the comparator is triggered
RANGE PIN CURRENT SENSING THRESHOLD
and turns off the bottom MOSFET. Ground 50mV Open 80mV When operated under light load, the inductor current goes V negative every cycle. The design of the inductor current IN 105mV ripple and the sensing resistor need to ensure that the
Slope Compensation
reverse current comparator is not triggered during normal operation. The LT3740 has a compensation slope to stabilize the constant-frequency valley mode operation. The slope
Power MOSFET Selection
compensation signal increases with the bottom gate duty cycle, which results in a current sense threshold voltage The LT3740 requires two external N-channel power change with duty cycle as shown in the fi gure in Typical MOSFETs, one for the top switch and one for the bottom Performance Characteristics. The three current limit levels switch. Important parameters for the power MOSFETs are correspond to three compensation slopes. the breakdown voltage V(BR)DSS, threshold voltage V(GS)TH, on-resistance RDS(ON), reverse transfer capacitance CRSS The compensation slope needs to overcome the difference and maximum current IDS(MAX). between the up and down slope of the inductor current to avoid sub-harmonic oscillation. Maximum compensa- When the bottom MOSFET is used as the current sense tion slope is required for high input voltages, where the element, particular attention must be paid to the initial varia- duty cycle is small. The compensation slope can only be tion, the gate-source voltage effect and the temperature selected by the RANGE pin. In the case of insuffi cient characteristics of its on-resistance. MOSFET on-resistance compensation slope, the inductor ripple current or the decreases as the gate-source voltage increases. The change sensing resistance needs to be reduced. of BGDP voltage could affect the bottom MOSFET gate voltage. Refer to the MOSFET datasheet for the MOSFET on-resistance corresponding to certain gate voltage. MOSFET on-resistance is typically specifi ed with a maximum value RDS(ON) at 25°C. In this case, additional margin is required to accommodate the rise in MOSFET on-resistance with temperature: RDS(ON) = RSENSE/ρT 3740fc 8
APPLICATIONS INFORMATION Current Sensing Range Reverse Current Limit
Inductor current is determined by measuring the voltage Because the LT3740 operates in forced continuous mode across a sense resistance – either the on-resistance of when the feedback voltage is higher than 720mV, the the bottom MOSFET or an external sensing resistor. The inductor current can go negative on occasion, such as maximum current sense threshold has three steps that are light load, shutting down with a slow SHDN signal, large selected by the RANGE pin. The current sense threshold load step-down transient response, or the output voltage voltage without slope compensation is shown in Table 1. being pulled up by some other power supply. The LT3740 This is the value for high duty cycle operation. has a reverse current comparator to limit the reverse current. During the on-time of the bottom MOSFET, when
Table 1. Current Sensing Thresholds
(VSN+)–(VSN–) reaches 40mV, the comparator is triggered
RANGE PIN CURRENT SENSING THRESHOLD
and turns off the bottom MOSFET. Ground 50mV Open 80mV When operated under light load, the inductor current goes V negative every cycle. The design of the inductor current IN 105mV ripple and the sensing resistor need to ensure that the
Slope Compensation
reverse current comparator is not triggered during normal operation. The LT3740 has a compensation slope to stabilize the constant-frequency valley mode operation. The slope
Power MOSFET Selection
compensation signal increases with the bottom gate duty cycle, which results in a current sense threshold voltage The LT3740 requires two external N-channel power change with duty cycle as shown in the fi gure in Typical MOSFETs, one for the top switch and one for the bottom Performance Characteristics. The three current limit levels switch. Important parameters for the power MOSFETs are correspond to three compensation slopes. the breakdown voltage V(BR)DSS, threshold voltage V(GS)TH, on-resistance RDS(ON), reverse transfer capacitance CRSS The compensation slope needs to overcome the difference and maximum current IDS(MAX). between the up and down slope of the inductor current to avoid sub-harmonic oscillation. Maximum compensa- When the bottom MOSFET is used as the current sense tion slope is required for high input voltages, where the element, particular attention must be paid to the initial varia- duty cycle is small. The compensation slope can only be tion, the gate-source voltage effect and the temperature selected by the RANGE pin. In the case of insuffi cient characteristics of its on-resistance. MOSFET on-resistance compensation slope, the inductor ripple current or the decreases as the gate-source voltage increases. The change sensing resistance needs to be reduced. of BGDP voltage could affect the bottom MOSFET gate voltage. Refer to the MOSFET datasheet for the MOSFET on-resistance corresponding to certain gate voltage. MOSFET on-resistance is typically specifi ed with a maximum value RDS(ON) at 25°C. In this case, additional margin is required to accommodate the rise in MOSFET on-resistance with temperature: RDS(ON) = RSENSE/ρT 3740fc 8
